For background, an antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are radiated into space. The antenna may also convert electromagnetic waves into electric current. The electric field or “E” plane determines the polarization or orientation of the radio wave. Most antennas operate using either linear or circular polarization.
A linearly polarized antenna radiates in one plane. In a circularly polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. If the rotation is clockwise in the direction of propagation, the sense is called right-hand-circular polarization (RHCP). If the rotation is counterclockwise, the sense is called left-hand-circular polarization (LHCP).
An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the “whip” antenna on an automobile. A horizontally polarized (linear) antenna has its electric field parallel to the Earth's surface. Television transmissions in the United States typically use horizontal polarization.
A rotational polarized wave radiates energy in both the horizontal and vertical planes and all planes in between. The difference, if any, between the maximum and the minimum peaks as the antenna is rotated through all angles, is called the axial ratio or ellipticity and is usually specified in decibels (dB). If the axial ratio is near 0 dB, the antenna is said to be circularly polarized. If the axial ratio is greater than 1-2 dB, the polarization is often referred to as elliptical. Circular polarization is an example of elliptical polarization, with an axial ratio of 0 dB. Circular is a nominal term in practice, as typical circularly polarized antennas are usually slightly elliptically polarized. Used herein, the term “circular” is intended to include elliptical.
Antennas are configured to receive and/or transmit a signal based on the polarization of the signal. In many instances, a single antenna is used to simultaneously receive and transmit different signals on orthogonal polarizations.
Many signals are used in communication systems and this requires diversity among the signals. One form of diversity includes spatial diversity. Spatial diversity includes separating antennas by some predetermined distance to assist in preventing interference from other signals. Interference can occur when a signal intended for one antenna is received by another antenna. An example of spatial diversity includes configuring an antenna structure to have multiple antennas spaced apart from one another for receiving and/or transmitting individual signals of a single polarization. Unfortunately, providing multiple antennas is costly and increases the physical load. Furthermore, antenna platform space is typically limited and spatial diversity increases the size.
So conventional approaches include multiple monopoles or dipoles with spatial diversity. Also, multiple monopoles are used with vertical or horizontal polarization diversity. A tri-pole antenna includes three dipoles oriented in three dimensions. If these conventional antennas provide multipath mitigation such as diversity reception, and/or multiple-input, multiple-output (MIMO), then they typically: may not be compact and ruggedized; may require some positioning and/or adjustment; may not be three-dimensionally omnidirectional; and may not be useable near or attachable to some surfaces.
Another form of diversity includes polarization diversity. Polarization diversity includes simultaneously radiating or receiving multiple polarizations, as a propagation path may fade in one polarization but not in another.
Another form of diversity includes angular diversity. Angular diversity includes directive receive antennas scanning at various look angles. In some instances, the direct patch may be blocked by obstructions, but an indirect path available at other directions, such as say a reflection from a water tower. Rich multipath environments may even offer multiple communications channels on the same frequency, as for example using different water tower or building reflectors.
U.S. Pat. No. 4,588,993, to Babij et al, describes an antenna array including 3 dipoles and 3 loops, centered and operated on the 3 principal axis and planes. The elements are described as being electrically small such that loop circumferences are much much less than 1 wavelength. The array is oriented towards instrumentation and resolving near fields. Rotational polarization and communications diversity are not addressed.
What is needed is a compact antenna for a relay node in a mobile network used, for example, during urban operations in a multipath environment. The antenna should preferably be small, lightweight and inexpensive. The antenna may be quickly deployable without the need to adjust the position, and it may be able to be attached or placed on any material. Such an antenna is preferably three-dimensionally omnidirectional, and may support multipath mitigation such as diversity reception, and/or multiple-input, multiple-output (MIMO).